Because of their high hardness, transition metal carbides are particularly useful in applications such as armour plating, blasting nozzles, mechanical seals, and cutting and grinding tools. Some of them are also used in the shielding and control of unclear reactors (B.sub.4 C) due to its neutron absorptivity, chemical inertness, and radiation stability, while the others are used to manufacture composites such as TiO.sub.2 -SiC.
One of the major problems associated with the use of transition metal carbides both in monolithic form or alloyed with other ceramics, is their low diffusivity and thus low sinterability. Their fabrication to full density requires very high temperatures and a powder of submicron size. Considering that the grain size in densified parts is determined by the size of the particles from which the parts are fabricated, it is advantageous to fabricate parts from powders comprised from fine, submicron-size particles.
Currently, the primary process for the production of titanium carbide powders involves the reduction of titanium dioxide (TiO.sub.2) with carbon source at temperatures in the range of 1700.degree. C. to 2100.degree. C. (Bit. Patent No. 811, 906, 1959).
The titanium carbide so produced has wide particle size range normally much greater than one micron due to high reaction temperatures and long reaction times. Further, undesirable inhomogeneities are frequently found in the stationary reaction mix due to diffusion gradients established during the reduction reaction. In essence, these inhomogeneities exist due to the lack of intensive mixing of the bed during the reaction.
Commercial production of boron carbide is most commonly carried out by the reduction of boron oxide with carbon in a batch electric furnace (U.S. Pat. No. 3,161,471). Because of the slow rate of heat conduction, the time required to complete the reaction is very long, on the order of days. The sintered mass of product that results from this process requires physical size reduction in order to achieve a particle size a small enough for densification. Because of the very high hardness of boron carbide, this size reduction step is extremely difficult and expensive and results in contamination of the product.
Another method for manufacturing boron containing ceramic powders is by reduction of boron oxide with magnesium metal, the so-called termite reaction (U.S. Pat. No. 2,834,651). In this process, reactants boron oxide, carbon, and magnesium are intimately mixed, loaded into a container, and the reaction initiated either by heating the reaction mixture to a sufficiently high temperature or through the use electrical igniters. This reaction is highly exothermic and self-propagating. The carbide powders produced by the termite process are unsatisfactory for high-purity applications because of contamination with magnesium.
In order to avoid these problems, researcher have sought methods of producing, high-purity, submicron size powders using direct synthesis from laser-heated or plasma-heated gases. Knudsen (Advances in Ceramics: Ceramic Powder Science 21, 237, 1987) described the synthesis of B.sub.4 C powders by the CO.sub.2 laser-driven pyrolysis of BCl.sub.3 /H.sub.2 /CH.sub.4 (C.sub.2 H.sub.4) gas mixtures.
Various other submicron-size boron containing powders, including titanium diboride (TiB.sub.2) have also been sinthesized from plasma-heated gases (U.S. Pat. No. 4,266,977). Although the laser or plasma heating process has a number of advantages such as fast heating rates, short reaction times and submicron particle size, the high cost of processing equipment, low production rates and expensive gaseous raw materials make this process commercially unattractive.